The naked and the dead earnest

Nakedness in pictures is like murder in fiction: a regular fixture and strangely normal. I mean that in life, in the life of this bit of the world, anyway, except in rather restricted circumstances, nakedness is a surprise. You don't expect to meet it in the street, in shops, the workplace, most bars, or while visiting friends - and this isn't a recent development.

But in pictures, nakedness, as such, is practically never a surprise. However slim the pretext, you don't go "oh, look, goodness, somebody with nothing on". And this isn't a recent development either. The world of art is a kind of nudist colony, in which the undraped figure may be expected at any turn.

It's true even today. The life class is hardly a thriving part of art education, and the conventions are thoroughly whacked ("a naked lady of uncertain age sitting on a kitchen chair" went a once-famous put-down, cheap but devastating). But the time-honoured association between art and the taking-off of clothes remains unbroken. "Nudity in Contemporary Art" would be a funny-sounding exhibition rubric, but there would be thousands of potential candidates. Perhaps a little history will be interesting.

"The Artist's Model: Etty to Spencer" isn't exclusively about nakedness. It's an exhibition about artists and models, in Britain, from the mid- 19th to the mid-20th century. Still, nakedness is obviously one of the things it focuses on, being one of the main things the artist's model offered. And the kit-on section of the show is fairly small. I suppose only about 20 per cent of the images got together at Kenwood House depict the fully dressed.

That may seem normal - as normal as that the majority of the models depicted are women, and almost all of them are human, and alive. But these things weren't always normal.

This show is really a sequel, and it needs its prequel to make these changes felt. In fact, it had one, but that was a while ago. The Artist's Model: Lely to Etty, went up (also at Kenwood) in the summer of 1991. I can just about remember it. One of its main points was that, in the traditional learning of the figure, looking at living, human bodies was a secondary business. The study of antique statues and anatomy came first. Also, the figure in question was normally male. The student was being trained up to paint ideal bodies in scenes of classical heroism.

That was the theory - and it had a remarkably good run, persisting long after it was clear nobody was likely to paint any such thing. In the current show, there's a Male Nude by the young Edward Wadsworth (later a leading Vorticist), done in the Slade life-room. The pose evidently relates to some high-minded neo-classical dilemma-tableau. The Choice of Hercules? The Continence of Scipio? But the date is 1911.

Still, that was a hangover. And the show traces the various changes in the use, status and image of the model. In life-rooms, there was an increasing use of women models, usually in inactive "female" poses - sitting around, lying around, standing to be seen. The female model also came to be imagined and portrayed as the male artist's muse and mistress. On the other hand, this was also the period when - after a lot of fuss - women were gradually allowed into life-rooms as artists.

So this exhibition provides a happy hunting ground for the student of gender, class, and sexuality. Of course, the evidence isn't always straightforward. Art is just about the last place you'd look for reliable information on the real lives of artists' models, and very few of these pictures are about their sitters as people. The model is partly fictionalised, standing for more or less than him/herself. I suppose that's a definition of a model.

It's a pity, with these absorbing topics, that there aren't better pictures to exercise them on. For instance, you can wonder whether Henry Scott Tuke's July Sun (1913) or Mark Gertler's Young Girlhood (1925) show signs of paedophilia, and answer yes, vaguely, and yes, certainly. But the one is too tepid and the other too superficial for the question to matter. The desires don't run deep - as they do in, say, Lewis Carroll.

There are some curiosities. John Tenniel (who illustrated Alice) has a watercolour of Pygmalion (1878) embracing his awakening statue, and gives her a prim, demure expression, as if being brought to naked life was an awkward social spot, to be handled as best one can. And Alan Beeton's 1929 scenes of a lay-figure acting as if alive are odd because they take a surreal sort of idea and paint it in a high-finish academic manner; the result is tweely horrible, rather like the ventriloquist episode in Dead of Night.

And there is some quality, too. James Etty, who died in 1849, is certainly a powerful artist, much criticised by his contemporaries for being lewd; his mixtures of the carnal and the statuesque are rare cases of raunchiness in English art. A big retrospective would be good. Or, for a chastening antidote, look at Dora Carrington's Reclining Female Nude (1911), which feels like it's been gently patted all over.

But very often with these naked models, the question is just: why?

Why paint this subject? Just because that's what artists do? The most exploitational motive would be better than none at all. And the most interesting pictures, I think, are those that take on this difficulty, that make the oddity of the nude their subject, and dramatise it, by painting the life class itself.

Take away the functional facts - some people are learning art and someone else is earning a little money - and it becomes a strange and ambiguous human transaction. Who is in charge? Who is in thrall? Everyone is fixed. The model sits, maybe high up, the centre of attention - but positioned, surrounded, trapped, a prisoner. Or are the students at their easels more like a congregation of votaries, obediently at their tasks? Victor Pasmore does a lovely half-comic glimpse of this static drama (1938), the draughtsmen suited and stiffly attentive, the model set above them, looking like she's kneeling on one of their heads.

But the most bizarre version, bringing out the quasi-sacred/sacrificial aspects of the art-rite in a really overt form, is an anonymous scene from the 1890s. The bearded model is doing Jesus for the room full of sketching students, dressed in a loin-cloth, and crucified. You don't see that everyday.

The Artist's Model: Etty to Spencer, Kenwood House, Hampstead Heath, London NW3; until 26 September; admission pounds 3.50. Then at Djanogly Art Gallery, University of Nottingham, 16 Oct to 12 Dec